1. Field of the Invention
The present invention relates to glass sheet heating furnaces of the elongated tunnel-type, wherein glass sheets are heated as they are conveyed therethrough upon a series of aligned rollers in preparation for subsequent treatment steps such as bending, tempering and annealing. More particularly, the invention pertains to such furnaces incorporating improved glass sheet heating and conveying means.
2. Description of the Prior Art
Glass sheets for use in the automotive industry are typically heated to their softening point, and then bent to a prescribed configuration dictated by the design of the automobile in which they are to be installed. After bending, the sheets which are to be employed as side lites and back lites are typically subjected to rapid chilling so as to develop a desired degree of temper for producing an acceptable break pattern. Sheets which are to be employed as windshields are gradually cooled so as to be annealed, and then laminated to another sheet or sheets in various combinations by means of a plastic interlayer.
Early glass sheet heating furnaces generally comprised an elongated refractory chamber of rectangular cross section through which the sheets were conveyed in succession upon a series of spaced metallic rolls. Heat was provided by suitable gas or electric heating elements positioned in the roof and side walls to, in turn, heat the interior of the furnace. The walls and roof were stationary members built up of refractory blocks. The rolls extended across the furnace and were supported in journals located outside the side walls. A worm gear affixed to one end of each roll drivingly engaged a worm wheel on a drive shaft running along and mounted for rotation adjacent a side wall of the furnace. Such conveyor systems, while being mechanically sound, were relatively expensive to build and maintain, and did not readily admit changing of individual rolls should that be necessary. In addition, the furnaces were not readily adapted to producing individualized heating patterns for glass sheets having different configurations and requiring different thermal characteristics.
Rolls having ceramic glass contacting surfaces were found to have significant advantages over metallic rolls in the conveying of the sheets through the furnace. Initially, the ceramic rolls were mounted and driven in a manner similar to the metallic rolls. However, it was found that such rolls more frequently require refurbishing to maintain their glass contacting surface in a suitable condition, so that it is occasionally necessary to remove and replace individual rolls or groups of rolls. This was very difficult and time consuming with the worm wheel and line shaft drive system, resulting in extended periods of lost production. In order to facilitate changing of the rolls, so-called friction drive systems were devised wherein the rolls rest at each end, outside the furnace enclosure, upon longitudinally extending belts. The ends of the rolls are restrained against longitudinal movement so that as the flights of the belts upon which the rolls rest are advanced, the rolls are driven in rotation. Changing of rolls and access to the furnace interior was further improved by the development of furnaces whose upper sections, that is, the portion above the roll line, comprises a vertically retractable assembly. In order to, among other things, improve heating efficiency and temperature control, it has been proposed to construct such furnaces in which either or both the lower and upper sections are of semi cylindrical configuration in cross-section.